This invention relates to a method and apparatus for controlling an adjustable speed electric motor and, more particularly, to a method and apparatus for vector control of an induction motor.
Electric power converters or inverters have been employed for the application of adjustable speed drives using alternating current motors. A typical converter includes a direct current (DC) rectifier for rectifying three-phase AC input voltage and for supplying the resulting direct current (DC) bus potential to an inverter. The inverter comprises a plurality of pairs of series-connected switching elements to generate an adjustable frequency output. In many applications, a frequency adjustment is effected through a control circuit which employs a pulse width modulation (PWM) control technique in producing variable frequency gating pulses to periodically switch the respective switching elements so as to operate the motor at a variable speed. The motor can be propelled (motoring mode) or retarded (braking mode) as desired by appropriately varying the frequency and the amplitude of the excitation that the inverter applies to the motor.
The actual motor speed is sensed and compared with a commanded motor speed. A speed error signal, which depends on the difference between the actual and desired values of motor speed, is derived and applied to a proportional-plus-integral control circuit which converts it into a torque command signal. The control circuit responds to the torque command signal by controlling the operation of the inverter so as to vary, as a function of the torque command signal, the amplitude of the voltages supplied from the inverter to the motor.
In order to provide more accurate motor control and linear motor torque control for variations in commanded torque, vector control has been proposed and employed. Such vector control utilizes a secondary flux rotational speed together with the torque command signal to control the momentary values of the frequency and amplitude of the stator current of the motor. It is current practice to calculate the secondary flux rotational speed by adding the sensed motor actual speed to a slip frequency calculated as a function of the torque command signal.
Accordingly, the conventional vector control requires a speed sensor positioned near the motor to sense motor rotational frequency. In addition, since the inverter control circuit is normally located at a long distance from the motor, a long code is required to connect the speed sensor to the inverter control circuit. Thus, the conventional vector control is subject to induction interference from motor currents or the like.